Citigroup acknowledged on Thursday that unidentified hackers had breached its security and gained access to the data of hundreds of thousands of its bank card customers.

“During routine monitoring, we recently discovered unauthorized access to Citi’s account online,” the bank said in an e-mailed statement. “We are contacting customers whose information was impacted.”

The giant bank said about 1 percent of its bank card holders had been affected, putting the total count of customers exposed in the hundreds of thousands, based on its annual report for 2010, which said its card business had about 21 million customers in North America.

While information concerning customers’ names, account numbers, addresses and e-mail addresses was exposed, the bank said that data like “social security number, date of birth, card expiration date and card security code were not compromised.”

“Citi has implemented enhanced procedures to prevent a recurrence of this type of event,” the bank said. “For the security of these customers, we are not disclosing further details.”

The Financial Times broke the story early Thursday.

Article source: http://feeds.nytimes.com/click.phdo?i=e5b18760f5e312a43a8a211ce4e132cb